1. Field of the Invention
The present invention relates to an image display device having a light emitting element, and more particularly, to a remedy of reduction in luminance of the light emitting element according to a deterioration of a light emitting material.
2. Description of the Related Art
A light emitting element represented by an electro luminescence (EL) element is high in visibility, since it emits light by itself. Therefore, the light emitting element is optimum to form in a thin shape since it does not need a backlight required for a liquid crystal display device. Further, a viewing angle of the light emitting element is not restricted. Therefore, in recent years, a display device using a light emitting element attracts attention as a display device substituted for CRT (cathode-ray tube) or LCD (liquid crystal display).
On commercialization of a display device using a light emitting element, however, a problem lies in the reduction in luminance of light emitting element which accompanies the deterioration of the organic light emitting material. Reduction in luminance makes the image blurred, and in the case of colorization, luminance differs depending on colors since each color is used for different time period. Therefore, a desired color cannot be displayed on a display device.
Accordingly, a method to keep the current flowing to the light emitting element constant to emit light is suggested. By controlling the luminance of light emitting element by current, change in luminance can be prevented.
FIG. 8 is a configuration example of a pixel in which a current is kept constant for light emission (Refer to Patent Document 1, for example). Connection of the pixel is described now. The pixel includes a first transistor (hereinafter referred to as Tr1), a second transistor (hereinafter referred to as Tr2), a third transistor (hereinafter referred to as Tr3), a fourth transistor (hereinafter referred to as Tr4), a fifth transistor (hereinafter referred to as Tr5), a light emitting element 809, a power source line 810, a data signal line 801 and a scanning line 802. Gate electrodes of Tr4 and Tr5 are both connected to the scanning line 802. One of a source region and a drain region of Tr4 is connected to the data signal line 801 while the other is connected to a drain region of Tr1. One of a source region and a drain region of Tr5 is connected to the drain region of Tr1 while the other is connected to a gate electrode of Tr3. Source regions of Tr1 and Tr2 are both connected to the power source line 810. A gate electrode of Tr1 is connected to a gate electrode and a drain region of Tr2. One of a source region and a drain region of Tr3 is connected to the drain region of Tr2 while the other is connected to a pixel electrode of the light emitting element 809.
As above-mentioned pixel is configured by a current mirror circuit, when Tr4 and Tr5 are ON, a current I1 flowing through Tr1 and Tr4 and a current I2 flowing through Tr2 and Tr3 are kept at the same current value. Moreover, the current I1 is controlled by the data signal line 801, which ends in controlling the current flowing to the light emitting element 809.
FIG. 4A shows I-V characteristics of Tr2 and Tr3. A current mirror circuit configured by one pair of transistors (one stage) shows a characteristic curve A, and a current mirror circuit configured by two pairs of transistors (two stages) shows a characteristic curve B. An advantage in configuring a current mirror circuit by two pairs of transistors is that an output resistance is large, for which a current can be kept constant in a saturation region. For example, even when a voltage VEL (voltage applied to the light emitting element) in FIG. 4B fluctuates and VDS (voltage applied to a transistor) is changed (it is assumed that |VDD−VGND| is constant), the current value can be kept constant in the case of characteristic curve B.
FIGS. 5A and 5B show an I-V characteristic in the case where an EL element and a transistor are connected in series. Tr2 and Tr3 are considered to be one transistor 501 with a large output resistance. FIG. 5A is a configuration diagram and FIG. 5B shows an I-V characteristic curve in the case where a voltage applied as a whole which is VDD=VEL+VDS is constant.
A voltage applied to a driver transistor and the EL element, and a current flowing to the EL element can be obtained at the intersection point (operation point) of two I-V characteristic curves. It is confirmed in FIG. 5B that the current value at the operation point is almost the same even when a characteristic of the EL element varies, that is when the characteristic curve of the EL element changes, as long as the driver transistor operates in a saturation region and an output resistance is large enough.
By setting the current value with the data signal line 801 while Tr1, Tr2 and Tr3 operate in the saturation region, a high definition display without luminance unevenness or display unevenness can be realized. It is to be noted that an EL element is taken as a representative of a light emitting element in this invention, however, the invention is not exclusively limited to the EL element.
[Patent Document 1]
                Japanese Patent Laid-Open No. 2002-251166        